A communication system is operable to communicate information between a transmitting station and a receiving station by way of a communication channel. A radio communication system is a communication system in which the communication channel by which information is communicated between the transmitting and receiving stations is formed upon a portion of the electromagnetic spectrum. A cellular communication system is exemplary of a multi-user, radio communication system.
Various cellular communication systems have been developed and implemented throughout large geographical areas. Cellular communication systems have been developed and implemented utilizing FDMA (frequency division multiple access), TDMA (time division multiple access), CDMA (code division multiple access), and various combinations of such communication techniques.
Communication systems utilizing CDMA communication techniques advantageously provide the possibility of increased communication capacity levels within a given frequency bandwidth allocated to the communication system. That is to say, CDMA communication techniques provide the possibility to transmit a plurality of signals simultaneously over a common bandwidth. Because of the simultaneous nature of communications in a CDMA communication system, particular attention must be given to the power levels of signals communicated on such shared bandwidth.
Gain control circuitry is utilized at a transmitting station to control the signal levels of signals transmitted therefrom upon the communication channel. And, the receiving station includes gain control circuitry for modifying the gain of signals representative of receive signals received at the receiving station. Gain control circuitry is utilized in transmitting and receiving stations operable in other communication systems, including the aforementioned cellular communication systems. And, gain control circuitry is also used in other types of devices, used for other purposes.
Receiver circuitry operable to process a signal transmitted upon a communication channel sometimes receives not only a desired signal component, but also interfering signal components. The interfering signal components might be of greater signal levels than the desired signal components. Signals transmitted during operation of a communication system constructed according to, and in compliance with, the standards set forth in the IS-98 specification pertaining to an analog/CDMA cellular communication system, promulgated by the EIA/TIA, states the interfering signal components to be as close as a 900 kHz frequency offset from the center frequency of a desired signal and as large as 71 dBc above the desired signal. This situation is commonly referred to as single tone desensitization.
When such a receive signal is processed at a receiving station and applied to a variable gain amplifier of variable gain circuitry, problems sometimes occur. A variable gain amplifier, typically forms a portion of the IF (intermediate frequency) portion of a receiver. The gain of the variable gain amplifier is controlled by application of a gain control signal thereto. If the gain control signal includes noise components, such noise components, when applied to the variable gain amplifier together with the receive signal, amplitude modulate interfering signal components of the receive signal. The result of such undesired modulation includes side bands which might obscure the desired portion, i.e., information portion, of the receive signal.
While efforts have been made to overcome this problem, such efforts have been constrained by the limited resolution permitted of existing processing circuitry. Such processing circuitry sometimes forms a portion of a gain control loop by which the gain of the variable gain amplifier is controlled. That is to say, data words generated by digital processing circuitry forming a portion of the gain control loop are of limited word lengths. And, digital-to-analog converters, used to convert the data words into analog form for application to the variable gain amplifier, are limited to 8-10 bits. Another manner is a pulse density modulator (PDM) which has a large switching component in its output.
A manner by which to prevent noise components of a gain control signal applied to a variable gain amplifier of a gain control circuit from interfering with operation of the variable gain amplifier would therefore be advantageous.
It is in light of this background information related to gain control circuitry that the significant improvements of the present invention have evolved.